1. Field of the Invention
The present invention relates to an axial flow fan, and more particularly relates to a noiseless axial flow fan.
2. Description of the Prior Art
Conventionally, an axial flow fan has been proposed as shown in the Japanese Patent Application Laid-Open No. 137297/94, wherein an outer peripheral edge of a blade is processed specially in order to reduce a noise of the fan.
FIG. 4 is a vertically sectioned side view of the conventional axial flow fan for explaining eddy currents of air to be generated at blade edges. FIG. 5 is a sectional front view of the blade shown in FIG. 4. A reference numeral 1 denotes a venturi case of the axial flow fan having a cylindrical cavity portion, 1-1 denotes an inner peripheral surface of the cavity portion of the venturi case 1, 1-11 denotes a suction port of the venturi case 1, 1-13 denotes a discharge port of the venturi case 1, 2 denotes an impeller driven by an electric motor, 2-1 denotes a plurality of blades of the impeller 2, 2-11 denotes an outer peripheral edge portion at the suction port side of the blade 2-1, 2-12 denotes an outer peripheral edge portion at an intermediate portion of the blade 2-1, 2-13 denotes an outer peripheral edge portion at the discharge port side of the blade 2-1, 2-14 denotes a negative pressure surface of the blade 2-1, 2-15 denotes a positive pressure surface of the blade 2-1, 3 denotes an air current, 4 denotes a leakage current of air, and 6 denotes a small air gap formed between the inner peripheral surface 1-1 of the cavity portion and the outer peripheral edge of the blade 2-1.
In the conventional axial flow fan shown in FIG. 4 and FIG. 5, the leakage current of air 4 directed from the positive pressure surface side to the negative pressure surface side is suppressed, so that the eddy currents of air generated at the blade edge in the small air gap is suppressed.
However, it is required to reduce more of the noise. In order to reduce the noise, it is necessary to reduce further the eddy currents of air to be generated in the gap between the inner peripheral surface of the cavity portion and an outer peripheral edge of the blade by reducing the leakage current of air flowing from the positive pressure side to the negative pressure side of the blade.
An object of the present invention is to provide an axial flow fan which solves the above tasks and problems.
Another object of the present invention is to provide an axial flow fan comprising a venturi case having a cylindrical cavity portion, an impeller having a plurality of blades, supported rotatably in the cylindrical cavity portion so that an outer peripheral edge of each of the blades faces to an inner peripheral surface of the cavity portion with a small air gap, and an electric motor for rotating the impeller, wherein the outer peripheral edge of the blade is increased in thickness gradually toward a discharge port side of the venturi case from a suction port side of the venturi case.
An outer peripheral edge portion of the blade at a suction port side of the venturi case is bent in the circumferential direction from a negative pressure surface side of the blade, so that the edge is small in thickness, and an outer peripheral edge portion of the blade at a discharge port side of the venturi case has a portion extending circumferentially from a positive pressure surface side of the blade so as to increase the thickness of the edge portion.
A circumferential length of an outer peripheral surface of the blade facing the inner peripheral surface of the cavity portion is small at the suction port side so that a leakage current of air flowing into a small air gap formed between the inner peripheral surface of the cavity portion and the outer peripheral edge of the blade is not prevented, but is increased gradually toward the discharge port side of the venturi case so that the leakage current of air flowing into the small air gap is suppressed gradually.